1. Field of the Invention
The present invention relates to a charging and discharging control circuit and a charging type power supply device, which are used for a secondary battery, and more particularly to a test function for evaluating a characteristic of the charging and discharging control circuit.
2. Description of the Related Art
A lithium ion secondary battery that has significantly contributed to the widespread use of portable devices, such as a mobile telephone and a personal handyphone system (PHS), is characterized by a small size, light weight, and a large capacity. Therefore, the long-time drive of the portable device and a reduction in weight thereof are realized. However, because the charging and discharging of the secondary battery are repeated, the possibility that the secondary battery will more likely be put in an over-charge state or an over-discharge state becomes higher. When the secondary battery is put in the over-charge state, a temperature of the secondary battery increases. Then, a gas is generated by the decomposition of an electrolyte solution, so that an internal pressure of the secondary battery increases or a metal of Li is precipitated. Thus, there is a fear of ignition or burst. In contrast to this, when the secondary battery is put in the overdischarge state, the electrolyte solution is decomposed to deteriorate the characteristic of the secondary battery. In order to prevent the occurrences of such states, a protection circuit is incorporated in a battery pack.
According to a fundamental technique of the protection circuit, a charging and discharging control switch circuit is provided on a charging and discharging path between the secondary battery and the main body of the portable device. When a charging and discharging control circuit detects a state in which the secondary battery is charged with a voltage equal to or higher than a predetermined voltage, a state in which the secondary battery is discharged with a voltage equal to or lower than a predetermined voltage, or a state in which the secondary battery is discharged with an excessive current, the charging and discharging control switch circuit is turned OFF to prevent the over-charge state, the over-discharge state, and an over-current state from occurring.
The lithium ion secondary battery has a high internal impedance. Therefore, a battery voltage appears to change according to charging and discharging currents. While the charging current is flowing, the battery voltage appears to be high. While the discharging current is flowing, the battery voltage appears to be low. When the battery is efficiently used, it is necessary to set an over-charge detection delay time and an over-discharge detection delay time. When false cancellation caused by a noise is prevented, it is necessary to set a cancel delay time. For example, JP 2001-283932 A (pages 1 to 6 and FIG. 1) discloses that the delay times are set by an internal delay circuit to shorten test times for over-charging and over-discharging. The internal delay circuit provides all delay times. Thus, it is unnecessary to provide external capacitors for determining delay times, so that the number of external parts of the protection circuit can be reduced.
However, in a charging and discharging control circuit using the internal delay circuit, the delay times are hard to change from the outside. The evaluation of the characteristic of the charging and discharging control circuit takes enormous amounts of test time because of the influence of the delay times. The over-current detection delay time and the over-discharge detection delay time each are generally about several milliseconds to 100 milliseconds, so that the test time is not significantly influenced by the delay time. Because the over-charge detection delay time is generally set to about several seconds, the test takes a long time. Therefore, it is necessary for the charging and discharging control circuit using the internal delay circuit to set a test mode for shortening the delay times.
JP 2001-283932 A discloses a charging and discharging control circuit and a charging type power supply device, in which a test mode for shortening the delay times in the internal control circuit is set when a voltage equal to or higher than a specified voltage is applied between a charger connection terminal. FIG. 2 shows such a circuit example. When the battery is put in the over-charge state, an output of an over-charge detection comparator 213 becomes a high level and an internal control circuit 220 outputs a control signal to an internal delay circuit 221. The internal delay circuit 221 receives an output voltage (control signal) from the internal control circuit 220 as an input signal and outputs a signal for controlling a switch circuit 202 after the lapse of a specified delay time t1.
When a voltage of an over-current detection terminal becomes equal to or higher than a specified voltage V1, an output of a voltage detection comparator 215 becomes a high level. When the output of the voltage detection comparator 215 is the high level, the internal control circuit 220 enters a state of outputting a control signal for shortening a delay time of the internal delay circuit 221, and holds the state. When the battery is put in the over-charge state, the output of the over-charge detection comparator 213 becomes a high level and the internal control circuit 220 outputs the control signal to the internal delay circuit 221. The internal delay circuit 221 receives the output voltage (control signal) from the internal control circuit 220 as an input signal and outputs the signal for controlling the switch circuit 202 after the lapse of a specified delay time t2. For that reason, once a voltage applied to the over-current detection terminal reaches the voltage V1 equal to or higher than the specified voltage, the short delay time is maintained. After that, an over-charge detection voltage can be measured in a state where the over-charge delay time is short.
When the voltage of the over-current detection terminal becomes equal to or lower than a specified voltage V2, an output of a voltage detection comparator 214 becomes a high level. When the output of the voltage detection comparator 214 is the high level, the internal control circuit 220 cancels the state of outputting the control signal for shortening the delay time of the internal delay circuit 221 to set the normal delay time t1. Therefore, once a voltage applied to the over-current detection terminal becomes the voltage V2 equal to or lower than the specified voltage, the internal control circuit 220 resets the test mode and returns to a normal state.
However, according to the invention in JP 2001-283932 A, it is necessary to detect the voltage of the over-current detection terminal at a plurality of levels. According to the above-mentioned technique, there is a problem in that a circuit structure is complicated and stable operation is hard to ensure.
In addition, when voltages for detecting the over-charge and over-discharge and for canceling are set in a mass production process, it is necessary to measure detection voltages with precision. At this time, a wait time equal to or longer than several seconds is required every time an input voltage is stepped up. If the detection voltages can be detected by 25 steps, a time required for measuring the detection voltages becomes 125 seconds in the case where the wait time is assumed to be 5 seconds. Even if the charging and discharging control circuit has a test mode for shortening the delay time to 1/50, the measurement takes 2.5 seconds per chip. Such measurement takes too long time in terms of mass production and becomes a serious problem in view of a test cost.
That is, in the case of initial measurement on a secondary battery charging and discharging control circuit including a delay circuit, which is performed at the factory, it is necessary to further shorten the detection delay time. In addition, in the cases of secondary measurement and customer's evaluation, both the normal use mode and the test mode for shortening the delay time are required. It is desired to realize such a control function using few external terminals.